Method of fabricating a reinforced tubular structure

ABSTRACT

A reinforced tubular structure comprising a metal tube and a cured composite tubular liner which increases the longitudinal stiffness of the tubular structure, the liner being adhesively bonded to the inner peripheral surface of the metal tube. The reinforced tubular structure is fabricated by forming a laminate of the composite material, wrapping it around a flexible mandrel, inserting the mandrel and wrapped laminate into the metal tube, and co-curing the laminate to the inner peripheral surface of the tube.

This is a division of application Ser. No. 16,454, filed Feb. 26, 1979,now U.S. Pat. No. 4,272,971.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

This invention generally relates to a reinforced tubular structure. Morespecifically, this invention relates to a lightweight reinforced tubularstructure and method of fabrication thereof, such structure having anouter metal tube and an inner composite tubular liner bonded to thetube.

(b) Description of the Prior Art

Metal tubular components of various shapes, i.e. round, rectangular,etc., such as drive shafts, tie rods, dead axles, cross members, andsteering mechanisms, are normally required to convey torque from a powersource to a means for converting this energy into useful work. Forexample, in the case of conventional automobiles, and other vehicles,the drive shaft conveys torque from the transmission to the differentialwhere it is converted into the drive force for the rear wheels of thevehicle.

Conventionally, such tubular structures such as automotive drive shafts,are constructed of steel, or similar dense material, and have asubstantial diameter and thickness in order to provide sufficientstiffness to meet the required torque and torsion requirements. Theexcessive weight of such shafts significantly increases the cost ofproducing and running the vehicle by increasing fuel consumption of thevehicle, reducing shaft critical vibration speed, and increasing cost ofthe shaft itself. Decreasing the thickness of the shaft to reduce weighthas not been a satisfactory solution, because while such a shaft couldcarry torsion loads, longitudinal stiffness would not be sufficient tomeet the drive shaft torque carrying and critical speed requirements.

The above considerations are even more significant in longer driveshafts such as those for long-bed trucks. Because of the lengthrequirement for the drive shafts, two or more drive shafts are usedbecause the weight/stiffness ratio of a single metal drive shaft wouldresult in too low a critical speed. The multiple drive shafts areconnected by a support bearing and frame structure. Use of the supportbearing and frame structure increases the weight and cost of the overalldrive shaft due to the extra parts required and labor for installationthereof.

To obviate the aforementioned difficulties, tubular structures, such asdrive shafts, have been fabricated using composite materials. Typically,these materials are formed of a resinous matrix reinforced with layersof filamentary material, such as Kevlar, boron, or carbon fibers.However, such composite tubular members have not been entirelysatisfactory. While light in weight and being able to provide shaftstiffness, composite tubular members have not been satisfactory withregard to carrying of torsion loads. In addition, composite material issubject to foreign object damage and environmental effects. In thisregard, chipping in the surface of the composite material caused byflying objects can cause delamination and effect an inbalance of theshaft. Environmental effects such as those resulting from moisture,chemical solvents, and heat can also result in delamination by breakdownof the adhesive bonding of the laminate. Connecting of composite tubularstructures to metal end members, such as for drive shafts, has alsopresented a problem. In addition, while users of vehicles (particularlytrucks) appreciate weight reduction of the vehicle, there is resistanceto visible non-metal substitutes for steel drive shafts (because ofconcern with foreign object damage and environmental effects on theshaft).

PRIOR ART STATEMENT

U.S. Pat. No. 3,458,374 to Shobert discloses a method for making abraided tubular bearing having a polytetrafluoroethylene liner. Thetubular structure formed by this invention is a plastic bearing which isformed around a lining of polytetrafluoroethylene.

U.S. Pat. No. 4,014,184 is directed to a propeller shaft liner wherein apaper tubular liner is inserted inside of the hollow cylindricalpropeller shaft. The paper liner must be of a flexible material toprovide resilient resistance to radial compression thereof. The liner isused to absorb and damp propeller shaft vibrations.

U.S. Pat. No. 4,089,190 to Worgan et al discloses an all composite driveshaft. U.S. Pat. No. 3,553,978 to Williams discloses a compositepropeller shaft. This shaft is made of a polyurethane foam arbor whichconnects end members, and a composite tube formed on the exterior of thearbor which engages the end members.

U.S. Pat. No. 3,372,462 to Reed et al discloses a method for makingplastic lined metal pipe. In this method, a plastic tube is heated toreduce its outside diameter, then inserted into a metal tube which hasits diameter reduced to allow contact with the outside diameter of theplastic tube, and then thereafter the assembly is cooled such that theplastic tube expands into tight engagement with the metal tube. Theplastic liner is disclosed to be used so that the pipe can be easilysterilized and so that other members can be attached to the pipe bybeing attached directly to the inner plastic tube rather than beingaffixed to the metal tube. It is not a purpose of this invention to usethe plastic liner for increasing the longitudinal stiffness of thetubular structure. In this regard, plastic materials, such as polyvinylchloride, which are capable of being reduced by heating, are used asopposed to a cured composite liner comprising a plurality of plies offibrous material in a solid resin matrix.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide areinforced tubular structure which is lightweight, yet economical,durable, and able to carry torsion overloads.

It is another object of the present invention to provide a reinforcedtubular structure combining features of light weight and high criticalspeed.

It is another object of the present invention to provide a reinforcedtubular structure which utilizes an inner composite reinforcing linerfor an outer metal tube.

It is another object of the present invention to provide a reinforcedtubular structure which utilizes a composite reinforcing liner which isnot outwardly visible, and where protection for the liner from foreignobject damage and environmental effects is provided.

It is another object of the present invention to provide a reinforcedtubular structure which obviates the need in long-bed trucks formultiple drive shafts and associated connecting hardware.

It is still another object of the present invention to provide a methodfor fabricating such a reinforced tubular structure.

Briefly, in accordance with the invention, there is provided areinforced tubular structure and a method for fabricating same. Thestructure comprises a metal tube having inner and outer peripheralsurfaces and a cured composite tubular liner within said metal tube. Theliner is adhesively bonded to the inner peripheral surface of the metaltube. The liner comprises a plurality of plies of fibrous material in asolid resin matrix. The liner increases the lonitudinal stiffness of thetubular structure. In the method of forming the structure, a laminate ofa plurality of plies of fibrous material in an uncured resin matrix isformed. The laminate is wrapped around a flexible mandrel. The mandrelwith the wrapped laminate is inserted into a metal tube having inner andouter peripheral surfaces. The laminate is then co-cured to the innerperipheral surface of the tube. Alternately, the laminate could be curedprior to insertion into the tube and bonded, rather than co-cured, tothe inner peripheral surface of the tube.

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a reinforced tubular structure inaccordance with the present invention is the form of a drive shaft;

FIG. 2 is a sectional view taken in the direction of arrows 2--2 of FIG.1;

FIG. 3 is an exploded perspective view of the laminate which forms theliner of the present invention with the plies thereof shown separatedsuch that the direction of the fibers of said plies are illustrated;

FIG. 4 is a perspective view illustrating the step in the fabrication ofthe drive shaft of FIG. 1 of wrapping the laminate around a flexiblemandrel;

FIG. 5 is a perspective view of the laminate fully wrapped around aflexible mandrel and back-up mandrel;

FIG. 6 is a fragmentary perspective view illustrating the step ofinserting the assembly of the wrapped laminate and mandrels into theouter metal tube;

FIG. 7 is a longitudinal sectional view of the wrapped laminate andmandrels inserted inside the outer metal tube, prior to expansion of theflexible mandrel and laminate;

FIG. 8 is a longitudinal sectional view illustrating the co-curing stepused in forming the reinforced tubular structure wherein the flexiblemandrel is expanded such that the laminate is compressed between theflexible mandrel and inner surface of the metal tube;

FIG. 9 is an exploded perspective view of the reinforced tubularstructure illustrating the end members which are attached to the metaltube; and

FIG. 10 is a perspective view of the reinforced tubular structureillustrating the step of welding the end members to the metal tube.

While the invention will be described in connection with the preferredembodiment; it will be understood that it is not intended to limit theinvention to that embodiment. On the contrary, it is intended to coverall alternatives, modifications, and equivalents that may be includedwithin the spirit and scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to FIGS. 1 and 2 of the drawings, there is seen areinforced tubular structure generally indicated at 10 according to thepresent invention. Tubular structure 10 is shown in the form of a driveshaft. Tubular structure 10 is comprised of a metal tube 12, end members14 and 16, and a cured composite lining 18 adhesively bonded to theinner peripheral surface 25 of tube 12. Metal tube 12 is shown ascylindrical, but can have cross sections of various shapes, i.e.rectangular. The metal used for tube 12 is typically steel as thisprovides, if in sufficient thickness and tube diameter, sufficientstiffness in both torsion and bending, protection from foreign objectdamage, and protection from many environmental conditions (such as heatand moisture). While steel is a preferred material, other metals such asaluminum or titanium could be used. End member 14 and 16 are welded, asdiscussed in more detail hereinafter, to metal tube 12 along areas 20and 22 respectively. Typically, end members 14 and 16 are adapted to beconnectable to universal joints for the receiving and transmission oftorque between members of the vehicle drive train.

The present invention obviates the aforementioned prior art problems byuse of a composite liner which reinforces the metal outer tube. Becauseof the increased longitudinal stiffness provided by liner 18, thethickness of the metal tube 12 can be reduced (as necessary to carrytorsion loads). As previously noted, torsion load requirements can bemet with a reduced thickness for metal tube 12, the torsion loads beingcarried through the welded joints of end members 14 and 16 and the metaltube 12. By virtue of the reduced thickness of metal tube 12, and theuse of the lightweight composite liner 18, overall weight of the driveshaft 10 is reduced. Thus, for a given length drive shaft, criticalspeed is increased. This allows with the present invention the use of alonger drive shaft while holding a given critical speed requirement,which can, depending on the overall length requirements, eliminate theneed in trucks for multiple drive shafts with the central supportbearing and frame structure.

Composite materials are normally strong, lightweight, tough,self-sustaining sheet material which are composed as a class, of aresinous matrix sheet reinforced with continuous, lineally aligned,parallel filaments. These sheets may be formed as a single layer sheetor as multi-layer laminates, and thereafter thermoset or cured to tough,hard, exceptionally strong panels.

As initially formed, these uncured sheet materials are flexible anddeformable, providing panel-forming members which can be draped orotherwise conformed to various shapes and thereafter cured, bythermosetting, uopon the application of heat and pressure thereto, totough, strong panels of permanent shape retention having exceptionaltensile strength and stiffness imparted by the continuous filamentreinforcing.

As shown in FIG. 3, liner 18 is formed of a laminate of a plurality ofsheets or plies. In FIG. 3, the laminate is shown as two sheets 30 and32. Sheets 30 and 32 are of uniform size and shape and are stacked tothe desired length to form the laminate (the laminate 18 can be seen inassembled condition in FIG. 4). Sheets 30 and 32 are formed from a resinmatrix reinforced with fibrous material in the form of continuous,lineally-aligned, parallel filaments 34. The sheets 30 and 32 areadhered together by the tackiness of the thermosetting resin matrixcontained in the material itself when acquired in pre-impregnated form(bonding of sheets 30 and 32 take place during the curing operation).

In keeping with the present invention, consideration must be given tothe fiber orientation in stacking the layers of the laminate. Optimally,the fibers or filaments 34 of sheets 30 and 32 are aligned such thatwhen the liner is positioned inside tube 12, the majority of fibers 34will be substantially parallel to the longitudinal axis of the tube 12.This is because the maximum stiffness provided by fibers 34 is in theaxial direction of the fibers. As such, maximum longitudinal stiffnesswill be provided to structure 10 by liner 18. In FIG. 3, the fibers areshown at an orientation indicated by arrows 36 and 38. Arrow 36represents an orientation of +5° whereas arrow 38 represents orientationof -5°. Normally, this would be with respect to an axis which isparallel to the parallel ends 40 and 42 of sheets 30 and 32. The reasonthat fibers 34 of sheets 30 and 32 are not parallel to each other at anapproximately 0° orientation, is that the cross-fiber orientation allowsthe fibers 34 of sheet 32 to support those of sheet 30 so that rollup ofthe laminate 18 is made easier and the fibers 34 are uniformlydistributed throughout sheets 30 and 32. However, depending upon themethod of formation of liner 18, fibers 34 of sheets 30 and 32 couldhave a 0° orientation. The important factor is that the majority(normally at least 80%) of fibers 34 have an orientation which issubstantially parallel, i.e. within 10°, to the longitudinal axis oftube 12. It may be desirable to have a small portion of the fibers, i.e.10%, at an approximately 90° orientation, i.e. positioned in asubstantially circumferential orientation around the liner 18 after theliner 18 is joined to tube 12. This would make the composite liner 18more compatible with regard to thermal expansion with metal tube 12. Assuch, a small sheet, or a plurality of small sheets, having 90° fibers,could be sandwiched between sheets 30 and 32.

As shown in FIG. 2, liner 8 has a variable thickness, being tapered fromthe middle to the ends thereof. This is basically due to thesubstantially trapezoidal shape (in plan view) of sheets 30 and 32 andthe wrapping of the laminate 18 around a mandrel 52 as shown in FIGS. 4and 5. This results in less and less of the length of laminate 18 beingoverlapped as it is rolled, i.e. there are more wrappings of thelaminate 18 towards the center thereof. The purpose of this as best seenin FIGS. 5 and 7, is so that the outer periphery of liner 18 isadhesively bonded to the inner peripheral surface 25 of tube 12 acrossthe windings of the laminate. This spreads the peel forces, which tendto separate the lining 18 from the metal tube surface 25, throughout thelaminate 18, and substantially eliminates notch effects which also tendto separate lining 18 from surface 25.

To provide the longitudinal stiffness reinforcement to the reducedthickness metal tube 12, it is important to consider the type ofcomposite material used for sheets 30 and 32, and specifically thefibers 34. The resin matrix can be of any suitable type and normallywould be either a polyamide or epoxy. In selecting the compositematerial for liner 18, it should meet the following test: ##EQU1## whereL and T are suscripts standing for liner and tube respectively, D equalsdensity, and E equals longitudinal Young's Modulus. The importantconsideration here is that increased longitudinal stiffness is providedto the metal tube 12 with less weight or density added to the driveshaft 10 than by a corresponding increase in thickness in the metal tube12. Thus, more stiffness is being added per unit weight with thecomposite liner than with the metal of the tube 12.

Optimally, the fibrous material will have a longitudinal Young's Modulusgreater than for the metal of the metal tube. Under such circumstances,the Young's Modulus for the liner would normally approximate that forthe metal tube. The advantage of course in using the high modulus fibersis that less plies would be needed for liner 18 to meet the neededlongitudinal stiffness reinforcement requirement. As such, cost andweight of the liner would be reduced (further, too low a modulus forliner 18 could necessitate a liner thickness greater than the insidediameter of tube 12, or not leave sufficient clearance for insertion ofthe liner 18 into tube 12). Some suitable fibers 34, where metal tube 12is steel, are listed below in Table I.

                  TABLE I                                                         ______________________________________                                                                YOUNG'S MODULUS                                                               (approximate)                                         FIBER  MANUFACTURER     (Pounds per Square Inch)                              ______________________________________                                        Pitch 50                                                                             Union Carbide    50 × 10.sup.6                                   Pitch 75                                                                             Union Carbide    75 × 10.sup.6                                   Pan 50 Union Carbide    50 × 10.sup.6                                   AS-1   Hercules Corp.   32-35 × 10.sup.6                                ______________________________________                                    

While only graphite fibers are listed above, other materials can beused, i.e. boron or KEVLAR. Steel has an approximate longitudinalYoung's Modulus of 29×10⁶ psi. In each of the above cases, the fibrousmaterial has a longitudinal Young's Modulus greater than for the steelof the metal tube 12. However, when the fibers are incorporated into theresin matrix, the Young's Modulus for the liner 18 is reduced from theabove figures. Table II sets forth the Young's Modulus for some of theabove fibers in an epoxy laminate, and the density of the laminate:

                  TABLE II                                                        ______________________________________                                               LAMINATE         LAMINATE                                                     YOUNG'S MODULUS  DENSITY                                                      (approximate)    (approximate)                                         FIBER  Pounds per Square Inch                                                                         Pounds per Cubic Inch                                 ______________________________________                                        Pitch 50                                                                             28 × 10.sup.6                                                                            .062                                                  Pitch 75                                                                             45-50 × 10.sup.6                                                                         .062                                                  AS-1   18-19 × 10.sup.6                                                                         .058                                                  ______________________________________                                    

Steel has an approximate density of 0.283 pounds per cubic inch. As anexample of the ratio test as described above, for the Pitch 50 fiber inan epoxy laminate, E_(L) divided by D_(L) equals 4.52×10⁸. This isgreater than the ratio of E_(T) divided by D_(T) for steel which equals1.02×10⁸. Normally, the liner ratio will be at least 400% greater thanthe metal ratio.

With further reference to FIG. 2, it can be seen that by welding the endmembers 14 and 16 to metal tube 12 with liner 18 provided inside ofmetal tube 12, the liner 18 is incased. The outer surface of drive shaft10 is all metal. As such, composite liner 18 is not outwardly visibleand is protected from foreign object damage, such as projectiles whichmay impact drive shaft 10, and from environmental effects such as water,and to a lesser degree heat, which can cause delamination of liner 18and separation of the liner 18 at the bond joint. It should also benoted that by using a non-metal material for liner 18, it has been foundthat the vibrations of the drive shaft are dampened.

The reinforced tubular structure 10 can optimally be formed as describedhereinafter. Referring now to FIGS. 3, 4, and 5, the uncured sheets 30and 32 are formed into a laminate 18. In the uncured state, compositesheets 30 and 32 are quite flexible and deformable. This allows theliner 18 to be rolled into a tubular configuration as shown in FIGS. 4and 5. In FIG. 4, laminate 18 is shown being rolled or wrapped around ahollow cylindrical flexible mandrel 52 on a flat surface, such as aboard 60. Mandrel 52 is preferably made of silicon rubber or othersuitable flexible material. Laminate 18 is wrapped around mandrel 52such that by virtue of the manner of wrapping and the substantiallytrapezoidal shape of laminate 18 (in plan view in unwrapped condition,or in other words, the shape of the principal surfaces of the laminatein unwrapped condition), a tubular structure with a variable thicknessfor laminate 18 results. In a typical wrapping of laminate 18, the outerends were four plies in thickness while the center portion wastwenty-eight plies thick. Normally prior to the wrapping step, flexiblemandrel 52 is placed around a hollow cylindrical back-up mandrel 54 suchthat mandrel 54 fits snugly and coaxially within the bore of mandrel 52.Mandrel 54 is preferably made of metal for rigidity. Joined to the endsof mandrel 54 are end fittings 56 and 58. End fitting 56 seals off thatend of mandrel 54 while end fitting 58 has a bore therein which allowsconnection to a line 86 for providing pressurized fluid, normally air,inside of mandrel 54. The fully wrapped laminate 18 is shown in FIG. 5.

Prior to insertion into tube 12 of the assembly generally indicated at65 of the wrapped laminate 18, flexible mandrel 52, metal back-upmandrel 54, and end fittings 56 and 58, the outer surface of the tubularwrapped laminate 18 is covered with an adhesive film (not shown),normally of the same material as the resin matrix used in the laminate,i.e. epoxy. This is an optional, but desired, step because the resinmatrix in the laminate itself would act as an adhesive during curing forjoining of liner 18 to tube 12, but extra adhesive on the bond surfaceincreases the strength of the bond. The film is then preferably treatedwith a substance which temporarily reduces the adhesiveness of theadhesive coating. An example of such a substance is powdered Cab-O-Sil.This substance is benignly absorbed into the adhesive coating during thecuring process without affecting bond quality. Also prior to insertionof the assembly 65 into tube 12, the inner peripheral surface 25 of tube12 is coated with a primer. Such a primer facilitates bonding of thecomposite liner 18 to the metal tube 12 and protects the metal fromoxidation during the bonding step. An example of such a primer is onesold under designation 3M EC3917 by Minnesota Manufacturing and MiningCorporation. The primer can be applied by spraying surface 25, dippingtube 12 inside a bath, etc.

FIG. 6 illustrates the step of inserting the assembly 65 into metal tube12. The diameters of mandrels 52 and 54 are such that the outer diameterof the wrapped laminate 18 at its largest point is slightly smaller thanthe inside diameter of metal tube 12. This assures that the wholeassembly 65 can be inserted into tube 12 without jamming. Assembly 65 ispreferably vertically inserted into metal tube 12 as this results in aminimum of contact pressure between the adhesive coating of the lining18 and the primed surface 26 of tube 12. Typically, this would beaccomplished by use of a vertical hoist (not shown) which is attached toloop 70 which normally forms part of end fitting 56 and a detachablehook 72. FIG. 7 illustrates assembly 65 as inserted inside of tube 12.

FIG. 8 illustrates the bonding or co-curing of the liner 18 to surface25 of tube 12. Back-up mandrel 54, which is preferably metal, has aplurality of apertures 80 (see also FIG. 7) in the surface thereof. Endfitting 58 has a cylindrical flange 82 at one end thereof. A threadedbore 84 is provided within flange 82. This allows the connection of line86 (FIG. 7), which has a protruding end 88 which is threaded on theoutside thereof for connection inside the threaded bore 84. Line 86 isconnected to a suitable source of pressurized fluid (not shown).Close-out sleeves 90 and 91, which act to seal flexible mandrel 52 toback-up mandrel 54, are placed around the ends of flexible mandrel 52which protrude from the ends of metal tube 12. Close-out sleeves 90 and91 are positioned adjacent to the ends of tube 12 such that duringexpansion of flexible mandrel 52, there will not be a gap between theclose-out sleeves and the tube ends. This would allow the rubber hose toexpand out through such gap which would likely cause a rupture ofmandrel 52. In order to assure that close-out sleeves 90 and 91 maintaintheir seal and are not pushed away from the tube ends 12, bolts 92 and93 are provided in sleeve 90 and bolts 94 and 95 are provided in sleeve91. Bolts 92 and 94 are connected to each other by a metal strap 100.Similarly, bolts 93 and 95 are connected to each other by a metal strap102. Straps 100 and 102 act to restrain longitudinal movement ofclose-out sleeves 90 and 91 away from each other. Pinch wires or "O"rings 110 and 112 are also utilized to assure sealing of the flexiblemandrel 52 to mandrel 54. "O" ring 110 fits between mandrel 52 andsleeve 90 while "O" ring 112 fits between mandrel 52 and sleeve 91.Sleeves 90 and 91 can be placed over "O" rings 110 and 112 respectivelyat room temperature because of an intentional loose fit. However, duringcuring and bonding, which takes place at elevated temperatures, therubber of the "O" rings 110 and 112 becomes hot and expands. Theexpansion tightens the fit of "O" rings 110 and 112 between theclose-out sleeves and flexible mandrel 52, thereby activating the seal.This type of seal allows the flexible mandrel 52 to expand in lengthwithout buckling the walls thereof internally.

Optimally, laminate 18 will be co-cured to the surface 25 of tube 12. Byco-curing is meant that the laminate 18 is cured while at the same timebeing bonded to surface 25. An alternative approach would be to cure thelaminate or liner 18, normally prior to insertion within tube 12, andthen bond the same to surface 25 of tube 12. However, this is notpreferred as an extra step is required and the fitting (and as such thebond) of the lining 18 to surface 25 would not be as good as withco-curing. During co-curing, the assembly (as shown in FIG. 8) is placedin a curing oven (not shown) where the temperature is raised to thecuring temperature, i.e. normally within the range of 250° F. to 600° F.and typically 350° F. for a graphite epoxy laminate. Once the assemblyis heated to the curing temperature, or simultaneously with the heating,pressurized fluid is introduced within mandrel 54. Such pressure wouldnormally be in the range of 45-100 psi. Typically, however, compressedair at about 85 psi is used. The compressed air within mandrel 54 passesthrough apertures 80 and in combination with the elevated temperatures,expands flexible mandrel 52 outwardly toward surface 25 of metal tube12. Expansion of flexible mandrel 52 accordingly expands liner 18outwardly and against surface 25 of tube 12. This expansion compresseslaminate 18 between mandrel 52 and surface 25 of tube 12. This conditionis maintained for approximately one hour whereupon liner 18 is cured andbonded (co-cured) to surface 25 of tube 12. By virtue of the flexibilityof the uncured laminate 18, the entire outer surface of the wrappedlaminate 18 contacts surface 25 even though the liner 18 tapers inthickness from the middle to the ends thereof. As such, the ends of thetubular liner 18 are expanded further to contact surface 25 than themiddle portion thereof. After curing, liner 18 still tapers in thicknessfrom the middle to the ends. However, it is the inner surface of liner18 which is tapered rather than the outer surface (see FIG. 2). If theliner 18 had already been cured prior to bonding to tube 12, this stepwould be the same except that all that would be accomplished is bondingof the liner 18 to surface 25, normally at the same temperature thatwould be used for co-curing, i.e. 350° F. for graphite epoxy, withflexible mandrel 52 providing the compressive pressure for bonding.

After the co-curing step, temperatures would be reduced to roomtemperature, pressure shut off, and the assembly as shown in FIG. 8removed from the curing oven. The close-out sleeves 90 and 91 and themandrels 52 and 54 could then be removed. As previously discussed, thefibers of liner 18 should be disposed substantially parallel to thelongitudinal axis of metal tube 12. At this time tube 12 would appear asshown in FIG. 9.

FIG. 9 illustrates the connction of end members 14 and 16 to tube 12. Ascan be seen in FIG. 9, a portion 120 at both ends of surface 25 of tube12 is not covered by lining 18. This allows for the end members 14 and16 which fit within the ends of tube 12, to be welded to tube 12 withmetal to metal contact. Thus, end members 14 and 16 would be welded toportions 120 of tube 12. This obviates the prior art problem of the weakmetal end member to composite tube connection. FIG. 10 illustrates thewelding of end member 14 to tube 12. As shown, a welding torch 130produces a welded joint 20 that fastens end member 14 to tube 12. Awater cooled split ring heatsink 134 is placed on the outer surface ofmetal tube 12 to isolate the welding heat from liner 18 (and the bondthereof to tube 12). After the end members are welded to tube 12, theliner 18 is incased, providing the aforementioned advantages.

Thus, it is apparent that there has been provided, in accordance withthe invention, a reinforced tubular structure and method of fabricationthereof that fully satisfies the objectives, aims, and advantages setforth above. While the invention has been described in conjunction withspecific embodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the aforegoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations thatfall within the spirit and scope of the appended claims.

What is claimed is:
 1. A method of forming a reinforced tubularstructure comprising:forming a laminate of a plurality of plies offibrous material in an uncured resin matrix, the laminate having twoopposed principal surfaces, the shape of the principal surfaces beingsubstantially trapezoidal; wrapping the laminate around a flexiblemandrel such that the laminate is formed into a variable thickness tubetapering from the middle to the ends thereof; inserting the flexiblemandrel into a metal tube, the metal tube having inner and outerperipheral surfaces; and co-curing the laminate to the inner peripheralsurface of the metal tube such that the laminate is bonded to the innerperipheral surface of the metal tube across a plurality of windings ofthe laminate, said flexible mandrel being expanded during said co-curingsuch that the laminate is compressed between the flexible mandrel andthe inner peripheral surface of the metal tube.
 2. The method of claim 1wherein the ratio of longitudinal Young's Modulus to density for thelaminate is greater than for the metal tube.
 3. The method of claim 2wherein the fibers of the fibrous material are disposed within thelaminate such that the majority of fibers will be aligned substantiallyparallel to the longitudinal axis of the metal tube after co-curing. 4.The method of claim 3 also including the steps of applying a primer tothe inner peripheral surface of the metal tube, and applying an adhesivecoating to the outer surface of the wrapped laminate.
 5. The method ofclaim 3 also including the step of connecting metal end members torespective ends of the metal tube such that the laminate is incased. 6.The method of claim 3 wherein the flexible mandrel is tubular and isexpanded by application of pressurized fluid therein.
 7. The method ofclaim 6 also including the step of placing the flexible mandrel around ahollow back-up mandrel, the back-up mandrel having a plurality ofapertures therein, and wherein the pressurized fluid passes from withinthe back-up mandrel through the apertures to within the flexiblemandrel.
 8. The method of claim 7 also including the step of removingthe back-up mandrel and flexible mandrel from within the metal tube. 9.The method of claim 8 also including the steps of applying a primer tothe inner peripheral surface of the metal tube, and applying an adhesivecoating to the outer surface of the wrapped laminate.
 10. The method ofclaim 9 also including the step of applying to the adhesive coating asubstance which temporarily reduces adhesiveness of the adhesivecoating.
 11. The method of claim 10 wherein the fibrous material isgraphite, the resin matrix is an epoxy matrix, and the flexible mandrelis made of rubber.
 12. The method of claim 10 also including the step ofconnecting metal end members to respective ends of the metal tube suchthat the laminate is incased.